This invention relates to printing methods, and more particularly to methods for printing halftone images on substrates having irregular surfaces and/or surfaces which are not of uniform conformity with the printing surface. This invention also relates to printing halftone images with apparatus in which high pressure is required for any reason at any ink transfer nip, and apparatus in which overinking is a problem. The invention has particular application to printing halftone images around truncated conical substrate surfaces such as the outer side surfaces of plastic containers.
Printing presses with cylindrical printing members have been adapted for printing on the outer side surfaces of plastic containers (e.g., cups) which have the shape of a truncated cone. In one common arrangement the already formed container is mounted on a rotatable mandrel and held so that the outer side surface of the container is in line contact with the cylindrical surface of the printing member. The printing member rotates about its longitudinal axis, thereby rotating the container and transferring ink from the printing surface to the container at the line contact or nip between the surfaces. Because the printing surface is cylindrical and the container surface has a truncated conical shape, the container surface is not uniformly conformable to the printing surface. Typically, the upper portion of the container, which has the larger circumference, has a higher linear velocity than the adjacent printing surface. The lower portion of the container, which has the smaller circumference, has a lower linear velocity than the adjacent printing surface. Only at some intermediate portion of the container is the linear velocity of the container surface the same as the linear velocity of the adjacent printing surface. Accordingly, the container surface is generally overfed near the top of the container and underfed near the bottom of the container. This causes circumferential elongation of the portion of the image near the top of the container and circumferential foreshortening of the image near the bottom of the container. Only the intermediate portion of the image is printed without distortion.
Not only are portions of the image distorted as described above, they are also frequently smeared or slurred. For example, the overfeeding of the top portion of the container surface tends particularly to slur the trailing edge of each feature of the image on that portion of the container.
Many printing substrates have localized non-uniformities which interfere with image transfer to them. For example, the wall thickness of thermoformed or molded plastic containers typically varies considerably. To insure good ink transfer to the container surface despite these surface variations or irregularities, substantial pressure is required between the printing surface and the container. Similar high pressure is required for satisfactory ink transfer to many other possible substrate materials with irregular surfaces such as corrugated cardboard, high basis weight cardboard, wood, nonwoven fabrics, kraft paper, polyethylene coated paper, and textured or embossed substrates such as embossed plastic film. Wherever such high pressure is required for good ink transfer, increased smearing or slurring of the printed image is frequently experienced.
Depending on the type of printing process involved, high pressure at ink transfer nips other than the nip at which the image is finally transferred to the substrate may also cause smearing or slurring of the printed image. In old or worn presses, high pressure may be required between the inking roller and the image cylinder to insure thorough inking of the image despite worn bearings, irregular surfaces, etc. If the image or plate cylinder is not used as the printing surface, the image must be transferred from the plate cylinder to a blanket cylinder which is then the printing surface. Again, high pressure may be required between the plate cylinder and the blanket cylinder for good image transfer to the blanket cylinder despite worn or irregular parts. High pressure at any of these ink transfer nips tends to cause slurring of the transferred image so that the final printed image is similarly slurred.
Overinking, which may occur occasionally in any printing operation and which is particularly common in old or worn presses, is another frequent cause of image smearing or slurring.
All of the foregoing problems are particularly aggravated in attempting to print small image details. Halftone images are made up entirely of small image elements and are therefore extremely difficult to print under the conditions described above. Image distortion of the kind encountered in printing or truncated conical surfaces such as plastic containers makes it very difficult to achieve uniform image density vertically on the finished container. The halftone image tends to be lighter or less dense than desired near the top of the finished container and darker or more dense than desired near the bottom of the container. Smearing or slurring of the image as a result of any or all of the above factors (i.e., non-uniform conformity of the substrate with the printing surface such as is experienced with conical containers, high pressure at any ink transfer nip, and/or overinking) also interferes with good halftone printing. The halftone dots are distorted by the slurring, thereby degrading the image. A small amount of distortion of each halftone dot has a large cumulative effect on the overall image. Intended levels of shading cannot be maintained and contrast may be lost. If the slurring is severe enough, the halftone dots may run together with the result that image details are completely lost.
All of the foregoing problems become even more severe in printing multicolor halftone images in which several monochromatic halftone images must be superimposed in proper registration and with proper density to achieve the desired composite result.
In view of the foregoing, it is an object of this invention to provide improved methods for printing halftone images on substrates having irregular surfaces and/or surfaces which are not of uniform conformity with the printing surface.
It is a more particular object of this invention to provide improved methods for printing halftone images on the outer side surfaces of truncated conical thermoformed or molded plastic containers.
It is another more particular object of this invention to provide improved methods for printing halftone images in any application in which high pressure is required at any ink or image transfer nip, or in which overinking is a frequent problem.